Optical study of defects in thick undoped CVD synthetic diamond layers

• Advanced optical imaging and spectroscopy techniques give insight into thick CVD diamond.
• 2 unique samples were grown, processed and characterized:
• The first one is a 760 μm thick CVD diamond film that exhibited a grey to greyish brown colour which is unusual for undoped diamond.
• The second one is an exceptionally thick CVD diamond crystal (3300 μm) also grown without any intentional addition of nitrogen.
• It was a unique test case where the characterization was performed after each processing step, meaning from as-grown to the polished outcome (round brilliant).
• Key information was obtained in a non-destructive manner with respect to the identification and distribution of defects, the crystalline quality and the relationship to bulk properties was acquired with micrometer resolution.

The growth of thick single crystal synthetic diamonds by plasma-assisted chemical vapour deposition (PACVD) that are colourless and with a low impurity content is an important challenge to achieve gem-quality material. To this aim, advanced optical imaging and spectroscopy techniques are useful tools to optimize the growth process as well as to identify CVD-made diamond gems. In this paper, two thick synthetic diamond crystals with unique structural and spectroscopic properties were grown by PACVD without intentional addition of nitrogen and without any post-treatment to enhance their colour. While the first one, sample A, was 760 μm thick and exhibited a grey to greyish brownish colour quite unusual for undoped material, the second one, sample B, was exceptionally thick (3300 μm) and colourless. It was laser-cut and polished to obtain a high gem-quality round brilliant of 0.44 carat with G colour and VVS2 clarity grade. The amount, distribution and nature of defects in these synthetic crystals is discussed using a broad range of optical characterisation techniques towards growth optimization and getting a better understanding of the material properties (and their possible integration to the gem market).